W210 K40 Relay Defective? Check Relay Unit A2105400072
Short answer: If your W210, SLK R170 or G-Class won't start when warm, sputters and stalls, or the fuel pump stays silent, the cause is often the central relay unit K40 in the front fuse and relay box – usually due to cold solder joints on the circuit board.
The solution is to swap it for the technically refurbished genuine K40 A2105400072 (re-soldered, function-tested, 5-year warranty) for 129,00 € – in stock with us.
The car runs perfectly when cold, but after the first leg of the trip and a short stop it no longer starts or simply stalls after a few meters. Half an hour later everything is back to normal. Anyone who now swaps parts on suspicion often misses the fault. On the affected Mercedes models, it is worth looking at the front relay box: that is where the relay unit K40 sits, through which the fuel pump, among other things, is switched.
Symptoms
First match your symptoms. The main indication of the K40 is that several functions switched through it are affected, or that the fault comes and goes depending on temperature.
| Symptom | Likely cause |
|---|---|
| Engine does not start when warm, stutters and stalls | Cold solder joint in the K40 – contact opens when warm |
| Fuel pump not working, no pump sound when trying to start | Pump relay or integrated 15 A fuse in the K40 |
| Starter motor does not turn even though the battery is full | K40 as a possible trigger; rule out battery and ground first |
| Fault disappears after tapping the module or after cooling down | Hairline crack in a solder joint on the circuit board |
| Only a single component not working, engine runs normally | More likely an individual plug-in relay than the K40 – see Check W210/W203 relay A0025421119 |
| Several comfort loads fail on the E 200 / E 230 | A different unit is installed there – see Check relay unit A2105400472 |
Causes
The K40 combines several relay functions of the vehicle electrical system in one pluggable unit. Depending on the model, it contains relays for fuel pump, electronic throttle (E-Gas), horn, ETS/ASR, secondary air pump and engine terminal 87, with their own fuses. The fuel pump runs through an integrated 15 A fuse. As a result, substantial currents flow continuously through a single circuit board.
Cold solder joints. The engine bay heats up and cools down on every drive. After two to three decades of temperature cycling, the solder joints at the relay and plug terminals crack in a ring shape. The contact holds when cold and opens when warm – which is exactly where the pattern “good when cold, failure when warm” comes from.
Aged components. Besides the solder joints, individual resistors on the circuit board also fail. In our refurbishments, defective resistors are therefore specifically replaced.
Testing it yourself
Work your way up from the cheapest cause. Disconnect the battery before working on the electrics.
- Battery and ground first. A weak battery or corroded ground points cause similar starting problems. Measure the resting voltage, inspect the ground points – it costs nothing.
- Read the number off the part. The K40 is located in the front fuse and relay box in the engine bay, behind the cover at the bulkhead. Note the complete number including the ending number – it determines the correct replacement part.
- Check the integrated fuse. Pull the 15 A fuse for the fuel pump and test it for continuity, don't just look at it.
- Tap test with warm engine. While the fault is occurring, lightly tap the K40 housing with a screwdriver handle. If the engine then starts or you hear the pump, a solder joint is the prime suspect.
- Swap with a tested part. The K40 is plugged in and can be replaced without tools. If the symptoms disappear with a tested unit with the same ending number, the cause has been found.
Solution
Once the K40 is confirmed as the cause, replacing it with a refurbished genuine unit is the reliable way. We have the central relay unit A2105400072 (also as A 210 540 00 72 written out, Stribel 89 8760 000) in stock: cold or poor solder joints re-soldered, defective resistors replaced, fully function-tested – for 129,00 € per piece, with a 5-year warranty, 60-day return policy and free shipping within Germany.
Mercedes K40 relay unit A2105400072
Part number: A2105400072 · Genuine part, technically refurbished
Fits: SLK R170 (pre-facelift), W210/S210, W463 from ident 106740 · 5-year warranty
129,00 €
Go to the spare part All relays & control units
If your part has a different ending number, you will find more units under Relays, Mercedes relays & control units and for the G-Class under W463 (G-Class).
Compatible vehicles
According to the parts data, the following vehicles are documented for the K40 A2105400072. On the SLK 200 (170.435) and the E 320 (210.055), several K40 versions were installed (00 72, 02 72, 04 72) – be sure to compare the number on your old part.
| Model series | Models | Note |
|---|---|---|
| SLK R170 (before facelift) | SLK 200 (170.435), SLK 200 Kompressor (170.445), SLK 230 Kompressor (170.447) | Not SLK from the 2000 facelift / SLK 320 |
| E-Class W210 sedan | E 320 with inline six-cylinder M104 (210.055), E 420 and E 50 AMG (210.072) | Not E 280/E 320 with V6 M112, not E 430/E 55 AMG |
| E-Class S210 T-Model estate | E 420 T (210.272) | Not W210 diesel |
| G-Class W463 from ID 106740 | G 320 with V6 M112 (463.232/233, convertible 463.209), G 500 with V8 M113 (463.241, convertible 463.206) | Note the ident cutoff |
Also not compatible, according to the parts data, are CLK W208, W202 and W211 as well as E 200 / E 230 (A 210 540 04 72 is installed there). If in doubt, we check the fit free of charge using your VIN (chassis number).
FAQ
How do I recognize a defective K40 relay unit?
Typically the engine no longer starts when warm, sputters and stalls, or the fuel pump does not work; a starter motor that does not turn can also be traced back to the K40. Because the faults often come from cold solder joints, they frequently occur sporadically and depending on temperature.
Does the K40 A2105400072 fit my W210, SLK or G-Class?
Documented are SLK R170 before facelift (SLK 200, SLK 200 Kompressor, SLK 230 Kompressor), W210 E 320 with M104, E 420 and E 50 AMG, S210 E 420 T as well as W463 from ident 106740 with G 320 (M112) and G 500 (M113). What matters is the number on your removed part – compare it 1:1.
Why is the ending number so important for the K40?
The versions 00 72, 02 72 and 04 72 look the same but have a different connector pin assignment. On the SLK 200 and the E 320, several versions were installed according to the catalog. A wrong ending number can therefore fail to work despite a matching housing.
What was done to the overhauled relay unit?
Cold or poor solder joints on the circuit board were re-soldered, defective resistors replaced, and the module was then fully function-tested. We give a 5-year warranty on this refurbished part.
How much does the K40 relay unit A2105400072 cost?
The technically refurbished genuine relay unit A2105400072 costs 129,00 € per piece, including free shipping within Germany and a 60-day return policy.
KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches. Every refurbished relay unit goes through a function test with us before it ships – over 1,000 positive eBay reviews speak for that. Unsure about diagnosis or ending number? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write to us via our contact form. We check the fit using your VIN (chassis number) before you order.